Winter
2000

Methods
to determine phosphorus loss from farm fields

by
John Sawyer, Department of Agronomy

Movement of phosphorus
(P) from farm fields to surface waters can elevate P in water systems
above critical levels for aquatic plant growth and thus enhance nutrient
enrichment and seasonal deficient oxygen, a process called eutrophication.
Phosphorus commonly controls vegetative production in freshwater bodies,
and hence the potential for eutrophication. The sourcing of P from production
fields (including P from manure and fertilizer) is now one focus area
considered as being an important contributor of total P entering surface
waters, and hence significantly contributing to water quality concerns.

Background.In April 1999, the Iowa Natural Resources Conservation Service (NRCS)
issued an Interim Conservation Practice Standard, Nutrient Management
Code (590). This standard is the guidance used by NRCS staff and the private
sector when providing technical assistance to producers requesting assistance
on nutrient management. Under some situations the technical guidance in
this standard may be required if the producer is voluntarily participating
in cost share programs that address water quality concerns. The NRCS in
each state is required to revise their state Nutrient Management standard
(590) in accordance with guidance provided by national policy and in the
national 590 standard. For P, the national standard provided states with
three options for guidance on application of P. In other words, there
is a choice of three methods states can use to assess the risk of P loss
from farm fields, and thus determine the potential management changes
needed to modify P application. This is a field-specific assessment of
the potential for P transport from the field. These options are 1) soil
test, 2) soil P threshold level, and 3) P Index rating. The state NRCS
has until April 2001 to implement one of these methods in the Iowa 590
standard.

Soil
test.This assessment method is very similar to an agronomic
interpretation of P need. The soil is tested using routine soil test P
methods for crop production, and test results are interpreted using tables
developed for crop response (Table 1). At soil tests less than optimal,
P is applied based on crop need (or at a nitrogen [N] need for the crop).
At some intermediate (optimal to high) level, P is applied based on the
crop removal. Eventually, P application is withheld at even higher soil
tests (Table 1, Excessive). The theory behind this risk assessment method
comes from the knowledge that as soil test P increases, dissolved P in
runoff increases.

This environmental
P interpretation does parallel agronomic use interpretation, like that
currently recommended in Iowa State University publication PM 1688,
General Guide for Crop Nutrient Recommendations in Iowa (for an example,
see Table 2 for corn P interpretation and recommendations).

Table
2. P recommendations for corn grain production.

P
Soil Test (ppm)

Soil Test
Category

Very Low

Low

Optimum

High

Very High

Bray P1
and Mehlich-3 P:

Low subsoil
P

08

915

1620

2130

+31

High subsoil
P

05

610

1115

1620

+21

Olsen P:

Low subsoil
P

05

610

1114

1520

+21

High subsoil
P

03

47

811

1215

+16

lb P2O5
to apply (lb/acre)*

100

75

50

0

0

*The recommended
amounts of P2O5 for the optimum soil test
category are based on nutrient removal for the reported yield.
The amount shown in the table for the optimum soil test category
is for 140 lb of corn grain per acre. Although P2O5
is not recommended at the high soil test category, a small amount
equivalent to that contained in 100 pounds of a common complete
NPK grade, applied as a starter fertilizer banded to the side
and below the seed row, may be advantageous under conditions of
limited soil drainage, cool soil conditions, or crop residues
on the soil surface. None is recommended for the very high soil
test category.

There are significant
advantages and problems in using the soil test approach to modifying P
applications for water quality purposes. Advantages include 1) uses soil
tests and sampling methods that are familiar to farmers and advisers;
2) follows agronomic guidelines for crop P need; and 3) applies simple
decision process and easy regulatory control. From the standpoint of optimal
agronomic and economic P resource use and protection of soil and water
resources, the soil test P risk assessment method makes a lot of sense.
Disadvantages include 1) research-based correlation between soil test
level and P reaching surface waters is limited; 2) management practices
(recent P application, rate, method, source, timing, and tillage) can
override the effect of soil test level on P losses; 3) beyond edge of
field management can affect P losses (distance to surface water, connectivity
between the field and water body, grassed waterways, and buffers); and
4) soil P tests do not predict soil erosion (P leaves fields in conjunction
with soil particles). There is also the issue of where and how to collect
soil samples for best prediction of P loss.

Phosphorus
threshold.This assessment method is very similar to the soil
test method. Instead of interpreting soil tests as given in Table 1,
and relating to crop need, an environmental soil P threshold level is
determined (Table 3). This environmental soil P threshold could be determined
from a routine soil P test, an environmental soil P test, P saturation
of the soil, or some other soil test. Advantages and disadvantages are
similar to those described for the soil test method. The largest disadvantage
is that no threshold value has been correlated to a critical P loss concentration
from farm fields (mainly due to the linear increase found in dissolved
P loss with increasing soil P level).

Phosphorus
Index (PI). The PI is an integrated approach to estimating
the risk of P loss from farm fields and movement to surface waters. Instead
of looking at just one test, it integrates the many field-specific factors
that influence P loss and potential movement to surface waters: erosion,
sediment delivery, relative field location in the watershed, buffer strips,
soil conservation practices, soil test P, precipitation, runoff, tile
flow, and P application (fertilizer or manure) method, timing, and rate.

The PI has several
advantages over other risk assessment methods: 1) estimates erosion and
sediment losses because total P is an important aspect of P supply to
surface waters; 2) accounts for beyond field edge effects on P reaching
surface waters; 3) includes P applications; and 4) adjusts for P management
strategies and soil conservation practices. The PI also could include
some characteristics of the other methods, for example, an environmental
P threshold. As for any of the P loss assessment methods, the predicted
risk of P delivery to surface waters indicated by a PI should be field
tested with representative situations (calibrated against measured P delivery)
and interpreted for surface water quality impacts.

The PI is more complex
and difficult to determine, but is a more reasonable and effective approach
to assessing risk of P loss from fields and delivery to surface waters
than soil test or threshold methods. Because of the integrated system,
the PI is useful for understanding the important factor or factors causing
a high P loss risk, and can help identify management practices to lower
that risk. And that is the goal, to reduce risks of P loss, help water
quality, and provide producers options for P management.

The
Iowa approach. The Iowa NRCS, through work and discussion
of the State Technical Committee, has decided from the three possible
methods to develop a PI for use in the Iowa 590 nutrient management standard.
Other midwestern states are also taking this approach. A PI is currently
under development in Iowa by a team of NRCS employees, Iowa State University
Extension specialists, and Iowa State University and United States Department
of Agriculture (USDA) soil scientists. Once recommended by the USDA State
Technical Committee and adopted by NRCS, an electronic version of the
Iowa Phosphorus Index and users guide will be available on the Web
at http://www.ia.nrcs.usda.go

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